Recently picked up a cracking VW LT35 motorhome which is fitted with the 6 cylinder 2.4L petrol engine (carburetor). It also has an LPG system fitted but I believe its running a bit lean. The power difference between petrol and LPG feels too large based on my previous experience with LPG and it'll backfire when on over run going down hills. The system is not what I'm normally used to either with a different switch over procedure which includes a physical switch for the petrol feed.

I thought I'd tweak the mixture screws on the LPG inlet however there aren't any. I seem to have a control link to some sort of lambda which was a surprise along with what looks like some form of solenoid on the gas feed to the carb inlet.

Any thoughts on what I have here and how to look at the lean running issues?

Hi,
That is an ld OMVL system called 'Econ EC' - you should be able to adjust the large brass screw on that actuator to control flow as well as the vapouriser in the normal manner. The little electronic box which is taped in place should give you a lambda reading which you can check for 'toggling' when adjusting.
from memory, there is also a vacuum connection to the actuator so worth double checking that.
Seem to remember it was quite a good (simple) controller and we might even have a few spares for it!

I'll try that fat little brass screw first. Yes there is a vacuum hose to the actuator. You can see it coming into the back of the solenoid under the rocker cover breather UFO thingy. It would be nice if that Lambda controller does give an actual live Lambda value. I had an AFR gauge hooked up to a Lambda on another conversion I had and it was a fantastic thing to have. Really nice to have a constant feedback on fueling.

Do you think there's a way of hooking up another gauge to this set up?

Another question. The two bolts on the vapouriser. Any idea which of these is the gas flow adjustment?

From the picture the vaporiser looks like an older OMVL R90E which would make sense as the rest of the system is OMVL. The smaller, top screw is the idle bypass which would normally be screwed fully home and the bottom, larger one is the main bias screw. Newer R90E vaporiser have two screws that are both 6mm hex and the other way round. The lambda controller should show lambda flicking from one end of the scale to the other when it is correct. If the leds in the display are different coloured, usually red is rich and green is lean. You need to set the vaporiser so that the lambda flicks at both idle and open throttle which is easier said than done with a lambda controlled system because changes you make may be adjusted by the controller.

Opened the larger screw on the vapouriser and that certainly increased fueling as revs increased. Then screwed the small screw on he vapouriser all the way in as suggested and then adjusted the lower screw until an ok idle was achieved.

Lights on the lambda controller were always hard to the left i.e. lean throughout this. Although I didn't check it with any higher revs. I should have and I will do shortly.

Anyway on a run power was certainly better. Still a little down on where I'd expect it to be and I'm still getting backfires on over run.

I think this needs a deeper look at now to be honest. I daren't adjust the vapouriser screw much further as its quite some way out now.

Anyone with any further thoughts? For example I note that this system only has a single pipe feeding the carb. A previous system I had with a much smaller engine had two.

Most only have a single pipe, only ones where the mixer has a separate idle feed use two, so that is normal. Do you know if the lambda sensor is actually working? When they die they give no output and as 0V = lean mixture it would show lean all the time no matter what it was actually doing.

Gilbertd wrote:Most only have a single pipe, only ones where the mixer has a separate idle feed use two, so that is normal. Do you know if the lambda sensor is actually working? When they die they give no output and as 0V = lean mixture it would show lean all the time no matter what it was actually doing.

No but certainly worth checking. I'll see if I can get a multimeter on a wire this week. I'm off work this week so I might have a chance to poke about.

If it has a carb then the lambda sensor has been added just for the LPG system and may well be the original one. It's probably a bog standard universal one. See what it shows when running on petrol, usually you would expect it to show rich at least under acceleration if not all the time.

The box (the electronic one with lambda control printed on it) should have some sort of display (I'd guess its a row of LEDS) on the top - At least I think thats what Charlie said further up in the thread. That should be showing the output fed to the controller with any luck if it works. Of course that relies on the wiring to the sensor actually working, which would cause same problem as a dead sensor, but at least should give you an idea of what the controller is seeing.

Brian_H wrote:The box (the electronic one with lambda control printed on it) should have some sort of display (I'd guess its a row of LEDS) on the top - At least I think thats what Charlie said further up in the thread. That should be showing the output fed to the controller with any luck if it works. Of course that relies on the wiring to the sensor actually working, which would cause same problem as a dead sensor, but at least should give you an idea of what the controller is seeing.

So far everything I've done is showing lean on the Lambda display. It hasn't moved when I've checked it.

So next check will be to see if there's any movement with the engine at higher revs. If not a check on any current from the lambda itself.

What does it show on petrol first? If the display is there then if its showing the same it does sound like thats where your fault is. Assuming its not down to a lack of fuel coming through of course, but the petrol test would tell you that (long as the controller is still live when on petrol of course, it may not be)

Brian_H wrote:What does it show on petrol first? If the display is there then if its showing the same it does sound like thats where your fault is. Assuming its not down to a lack of fuel coming through of course, but the petrol test would tell you that (long as the controller is still live when on petrol of course, it may not be)

Very good point. For some reason I didn't think it would be live when on petrol but you're right it should be showing something regardless. I'll check

If it isn't live when on petrol, connect a meter. The lambda sensor actually generates a voltage. They are described as 0-1V sensors but in reality you'l get an output between 0.1 and 0.9V depending on whether running lean or rich. A carb will go rich on acceleration due to the accelerator pump. If you got 0V all the time, it's dead.

Also possible the sensor is working, but the heater isn't working. So you won't get any codes for it if so as its not part of the petrol system anyway. And probably nothing from it on idle. You should be able to test that with a meter (resistance across the heater circuit). Might be an idea to have a look and see what sensor it is if you can identify it while your there, hopefully its a common sensor to find a replacement one for it.

That assumes it's a 3 or 4 wire sensor. On an old system it could even be a single wire one. A single wire sensor has no heater and is grounded through the body, a two wire also has no heater but has a signal and ground wire. 3 wire has two heater wires, a signal wire but no signal ground and finally a 4 wire has signal, signal ground and two heater wires. Colour codes for the different sensors are shown here http://www.lambdasensor.co.uk/main/mcolours.htm which should at least help you identify what is fitted.